1. Field of the Invention
The present invention relates to improved inorganic pigments coated with ZrO.sub.2, SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2, SnO.sub.2 or CeO.sub.2 or mixtures thereof and a method for multi-coating inorganic pigments to achieve good durability, low photochemical and chemical reactivity, and good optical and dispersibility properties in plastic and paint applications. Most specifically, the present invention relates to an improved titanium dioxide pigment having a first coating of Al.sub.2 O.sub.3, SiO.sub.2 or optionally other inorganic coating material or mixtures thereof, and an optional coating of ZrO.sub.2, SiO.sub.2, TiO.sub.2, SnO.sub.2, CeO.sub.2 or mixtures thereof over the first coating and a final coating of Al.sub.2 O.sub.3.
2. Description of the Prior Art
The surface treatment and coating of inorganic pigments, such as titanium dioxide for example, to achieve improved performance characteristics in plastic compositions, is known in the art. Dispersibility, processibility and good optical properties are typically achieved through minimal surface treatment of the pigment, whereas durability and low chemical and photochemical reactivity is typically obtained by coating the pigment with silica, alumina, and other metal oxide coatings. When precipitated using prior art practices, these metal oxide coatings tend to make the pigments more difficult to disperse in plastics or paints. Typically, these problems have not been solved with a single pigment. Thus, plastics compounders have generally had to choose between durability, which can be achieved with coated pigments, and dispersibility, processibility and good optical properties, which can be achieved with treated and untreated pigments. Similarly, paint performance can be improved through conventional surface treatments, but adding the first treating agent prior to crossing the isoelectric point maximizes both durability and dispersibility.
For example, U.S. Pat. No. 3,523,810 discloses a process for coating titania pigments with from 0.5 percent to 10 percent boehmite alumina at a pH of from 7 to 9 at temperatures between 70 and 100.degree. C. A pigment of this type with only boehmite and no amorphous alumina may have good optical and dispersibility properties but does not achieve good durability. Similarly, U.S. Pat. No. 4,022,636 describes the addition of two coatings of alumina to titanium dioxide pigments, one amorphous coating at a pH of about 2 followed by a boehmite coating at a pH of about 8, to achieve a pigment with good performance in plastics. Also, U.S. Pat. No. 4,416,699 describes a method for coating a pigment with between 2 percent and 6 percent alumina to achieve the chalk-fade resistance, gloss, and dispersibility required for paint grades of titanium dioxide. However, this process contains higher alumina levels than are permissible for optimum performance of titanium dioxide pigments in plastic applications.
Examples of inorganic pigment coating processes to which improved performance benefits have been ascribed include the coating process disclosed in U.S. Pat. No. 2,671,031. This patent teaches the precipitation of a hydrous aluminum oxide coating upon titanium dioxide followed by heat treatment between 500.degree. C. and 800.degree. C. to decrease pigment chalking. U.S. Pat. No. 2,780,558 discloses a process for coating pigmentary materials with alumina added as an alcoholate compound. In U.S. Pat. No. 3,146,119 the claimed process requires the precipitation of a layer of titanium dioxide onto the pigment prior to the precipitation of an alumina layer. U.S. Pat. No. 3,409,454 discloses an alumina-coated titanium dioxide pigment and process for obtaining the same wherein the alumina is precipitated onto the pigment under alkaline conditions (pH greater than 10) for improved pigment dispersibility in plastics. U.S. Pat. No. 3,529,985 discloses a double coated titanium dioxide pigment wherein the first metal oxide coating is calcined at a temperature of at least 450.degree. C. before addition of the second coating for improved durability and optical properties. U.S. Pat. No. 3,853,575 discloses the production of alumina-coated titanium dioxide pigments wherein the alumina coating is supplied by the addition of an alkaline alumina salt to an aqueous titanium dioxide solution. U.S. Pat. No. 4,239,548 discloses a titanium dioxide pigment with improved photochemical stability achieved by a first coating of cerium and phosphate radicals onto the pigment and a second coating of aluminum and phosphate radicals over the first coating. U.S. Pat. No. 5,114,486 discloses a process for coating titanium dioxide pigments with zinc, phosphate, and aluminum ions for improved processibility.
None of the aforementioned patents suggest or teach the coating of inorganic pigments with a first layer of Al.sub.2 O.sub.3 SiO.sub.2 or optionally other inorganic material or mixtures thereof, and an optional layer of ZrO.sub.2, SiO.sub.2, TiO.sub.2, SnO.sub.2, CeO.sub.2 or mixtures thereof and a final coating of Al.sub.2 O.sub.3, to produce the coated pigments which are the subject of this application.